Hmmm, walking speed (say 1-3 mph) as less than 2 rps. That says you've already chosen your wheels and done some arithmetic.
For a mobot (mobile robot) in the 150 lb. class the first thing that comes to mind is powered wheel chair motors. Check your yellow pages (paper or electronic) for wheel chair providers. You might find a set for cheap if you do the repair/rebuild (or even free dumpster diving).
High torque/low speed is usually achieved with FET pwm controllers. Two providers are vantec and 4qd; there are many others.
Accurate speed control relies on good feedback. Encoders on the wheels is a common method. This also allows steering by comparing wheel speed.
Compact, light-weight batteries just about mandates Li-ion. One alternative to look at is fuel cells.
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